Recently, behavior of vortex (quantized magnetic flux) in the superconducting state of high-purity niobium has drawn attention practically, because it is recognized to be a key factor to improve the quality factor (Q-factor) of superconducting radio-frequency (SRF) cavities for the acceleration of charged particles, because dissipation in the normal core of vortices cause a residual surface resistance even at very low temperatures (~ 2 K). Therefore, perfect removal of vortices from a SRF cavity will lead to a higher Q-factor. In addition, suppression of the energy consumption by the dissipation in SRF cavities is also helpful to reduce the cooling cost. To visualize the behavior of vortices in high-quality niobium samples (RRR~300), which are the same grade as one used for the SRF cavities, we have conducted the observation of local magnetic fields on a mirror-polished surface of niobium samples by a magneto-optical imaging (MOI) technique. MOI can visualize a magnetic field distribution on a surface of superconducting materials in real time, and even resolve individual vortices at its best. Although we have not seen individual vortices yet, we could confirm the formation of the vortex bundles, which is expected in high-quality niobium of a type-II/1 superconductor. It is important to study properties of the vortex bundles and whether the bundle formation influences on the vortex expulsion in a cooling process. In our presentation, we will show the formation process of the vortex bundles in a field-cooling, magnetic field dependence of the bundle size and shapes, and a vortex phase diagram.

Keywords: Vortex Bundle, Niobium, Magneto-optical imaging